[FFmpeg-user] forcing bitrate?

[Langdon]

is there a way to force the bitrate when extracting audio from a video?

I tried -b:a 1536k, but ffmpeg will force it to 1536.2kbps, which seems to
affect the duration of the sound clip I get back (seems to add silence to
the beginning).

After doing some research it appears that maybe the video wasn't clipped on
a GOP boundary... if that's the case am I out of luck for any easy fix?
http://ffmpeg-users.933282.n4.nabble.com/inaccurate-splitting-of-PCM-audio-td935314.html

TIA